High-pressure liquid propellant ducts on rocket engines are generally heavy and can represent approximately 10-20 percent of the total weight of the engine. Generally, the ducts comprise a tubular section that is joined to a flange portion at each end thereof to form a section of high-pressure liquid ducting. The section of high-pressure liquid ducting is further joined to another section of ducting or to another component of the rocket engine such as a propellant feed tank or an inlet to the rocket engine of, for example, an aerospace vehicle.
In the known art, high-pressure liquid propellant ducts are fabricated from nickel or iron-based superalloys such as 625 or 718, which have relatively high densities and thus contribute significantly to the weight of a rocket engine. To minimize weight, therefore, the ducts are ideally fabricated from materials with high specific strength and toughness for the tubular sections, which generally perform as pressure vessels. Similarly, the flange portions are fabricated from materials with high specific stiffness and hardness, wherein the flange portion performs primarily as a sealing section.
At the ambient or cryogenic temperatures of typical liquid propellant ducts, superalloys do not have an ideal high specific strength for the tubular section or an ideal high specific stiffness for the flange portion. For example, when a superalloy tubular section has been designed with sufficient strength to accommodate propellant pressure, the associated stiffness is relatively high such that small misalignments of the flange portions results in high stresses therein after assembly of the duct. Further, the assembly stress can represent approximately 60 percent of the total flange loading. Moreover, welding, weld inspection, and any rework necessary during assembly of high-pressure liquid ducts of the known art adds substantial cost to the rocket engine.
High strength aluminum alloys, although approximately one third the density of superalloys, have not been used for propellant ducts for a variety of reasons. Generally, the high strength condition in conventional aluminum alloys is achieved by a solution heat treat, followed by a water quench and age, which introduces constraints on forming, welding, and maximum component section thickness. Unfortunately, the aluminum alloy comprises undesirable residual stresses, anisotropic properties, and susceptibility to stress corrosion as a result of the heat treat, water quenching, and aging processes.
Further, conventional aluminum alloys generally have low stiffness, and thus any potential weight benefits of high specific strength in the stiffness-critical flange portions have not been achievable. Moreover, the high coefficient of thermal expansion of conventional aluminum alloys would require an excessively high preload in steel or superalloy bolts that are used to fasten and seal the flange portions to prevent loosening of the bolts during a chill-down process.
Accordingly, there remains a need in the art for lightweight high-pressure liquid ducts comprising aluminum alloys to provide significant weight savings over superalloy ducting of the known art. The high-pressure liquid ducts should comprise lightweight tubular sections in addition to lightweight flange portions, which are fabricated and assembled using manufacturing techniques applicable to the particular materials employed throughout the ducting.